1. Field of the Invention
The invention herein relates to a chemical reactor and method for catalytically reducing the content of nitrogen oxide in a gas, particularly flue or stack gas, resulting from the combustion of fuel.
2. Description of the Related Art
The combustion of fuels in various industrial processes often generates undesirable oxides of nitrogen (NOx), usually in the form of nitric oxide (NO) and nitrogen dioxide (NO2). High combustion temperatures tend to produce more NOx. Because NOx is harmful to the environment, efforts have been made to reduce the emission of NOx. in gases produced by industrial processes involving the combustion of fuel, particularly gases resulting from the operation of power plants, thermal cracking furnaces, incinerators, internal combustion engines, metallurgical plants, fertilizer plants and chemical plants.
Methods for selectively reducing the NOx content of a flue gas are known. Generally, such methods involve the reaction of NOx with a reducing agent, optionally in the presence of a catalyst. The selective non-catalytic reduction (xe2x80x9cSNCRxe2x80x9d) of NOx with a reducing agent such as ammonia or urea requires a relatively high temperature, e.g., in the range of from about 1600xc2x0 F. to about 2100xc2x0 F.
Alternatively, the reduction of NOx with ammonia can be performed catalytically at a much lower temperature, e.g., from about 500xc2x0 F. to about 950xc2x0 F., in a process known as selective catalytic reduction (xe2x80x9cSCRxe2x80x9d).
One problem associated with the treatment of flue gas using conventional SCR methods and apparatus is that the weight and bulk of the equipment necessary to achieve satisfactory removal of NOx requires that it be located at ground level. Many industrial plants need to be retrofitted with NOx removal (xe2x80x9cdeNOxxe2x80x9d) equipment in order meet the requirements of more stringent government regulations. However, because of the physical bulk of the deNOx system, the flue gas must be diverted to ground level for treatment and then sent back into a stack for subsequent exhaust to the atmosphere. To avoid the large cost of such a system it would be highly advantageous to provide a relatively lightweight deNOx unit which can be incorporated directly into the stack.
In accordance with the present invention, a parallel flow gas phase reactor is provided for the chemical conversion of nitrogen oxide in a gas stream and comprises:
a) a shell having interior and exterior surfaces, a gas stream inlet for receiving an inlet gas stream having an initial concentration of nitrogen oxide and a gas stream outlet through which treated gas of reduced nitrogen oxide concentration relative to the nitrogen oxide concentration of the inlet gas stream is discharged;
b) an injector for introducing a reducing agent into the inlet gas stream; and,
c) a plurality of substantially planar catalyst beds within the reactor shell, each catalyst bed containing at least one nitrogen oxide conversion catalyst for the selective catalytic reduction of nitrogen oxide in the inlet gas stream to provide a treated gas of reduced nitrogen oxide concentration, the catalyst beds being oriented substantially parallel with, and in spaced-apart relationship to, each other and to the interior surface of the reactor shell with gas flow passageways therebetween, the passageways each including a gas stream deflector positioned therein for directing the flow of inlet gas stream through at least one catalyst bed and treated gas to the gas stream outlet, each catalyst bed being a monolith or catalyst supported on a mesh-like structure having a porosity greater than about 85%.
The parallel flow reactor of this invention provides a relatively lightweight unit for the selective catalytic reduction of NOx in a gas, in particular flue gas produced by the combustion of a fossil fuel in a furnace, and is readily incorporated into furnaces equipped with stacks of conventional design, thus lending itself well to retrofit installation in existing units.